This invention relates to electrode devices which are capable of accepting small volumes of samples, and to their use in a test method for the detection and quantification of a test species present in a small sample volume.
Many devices have been disclosed, that are capable of accepting small volumes of sample material, and that allow analytes present in the sample to be interrogated, either by optical or electrical analytical processes. In particular, the use and construction of sample chambers capable of filling by capillary action has been described in both the patent and scientific literature. See, for example, EP-A-0170375 and US-A-5141868.
Such known devices may comprise electrodes deposited on a non-conducting substrate, coated with a reagent system specific for the analyte of interest and housed within a cavity whose dimensions are sufficiently small to allow introduction of a sample by capillary action. The sample is retained in close proximity to the electrodes, and the electrodes are configured in such a way as to facilitate the measurement of specific electrical properties of the sample.
Such devices suffer from numerous drawbacks, in particular the need to control the dimensions of the cavity within very tightly defined limits. Exceeding these manufacturing tolerances will prevent the sample from entering the cavity by capillary action.
Further, when viscous sample fluids such as blood are introduced into the cavity, the chamber will fill with sample relatively slowly, thus delaying the time taken-to complete the analysis. Variations in sample viscosity and thus sample surface tension characteristics result in variations of the fill time; this not only compromises the overall analysis time but, more importantly, leads to imprecision in the analytical result, since the time over which the sample is exposed to the analyte-specific reagent is subject to variation.
WO-A-9730344 discloses an electrode device which includes a polyester mesh adapted to guide the sample to the reference electrode. This device requires that the reagent includes a filler having both hydrophobic and hydrophilic surface regions, in order to avoid problems associated with variations in sample handling and to be independent of the haemocrit of the sample, for glucose testing.
According to the present invention, a device which is capable of electrochemical measurement of the levels of analytes present in a small fluid sample volume, comprises a conductive layer coated with an analyte-specific reagent and deposited on a non-conducting substrate, a spacer layer deposited onto the non-conducting substrate by thick film printing, a monofilament mesh material coated with a surfactant and/or a chaotropic reagent, the mesh being overlaid onto the space layer, and a second non-conductive substrate adhered to the mesh layer. The device is thus multilayer in construction, and comprises two surfaces separated by a printed spacer layer and forming a cavity or area which is open at one end for the introduction of sample. This cavity or area is filled with a mesh material that extends beyond the second substrate and forms a sample application area.
A device according to the present invention may be produced and used by the steps of
(a) depositing a conducting layer of carbon and graphite, in a polymer binder, on a first non-conducting substrate;
(b) depositing a second conducting layer consisting of silver/silver chloride to function as a reference/counter electrode, adjacent to but not continuous with the first conducting layer;
(c) coating the surface of the first conductive layer with a reagent or mixtures of reagents which react specifically with an analyte or analytes in a sample material;
(d) forming a spacer layer by thick film printing on top of the first non-conducting substrate and on top of the first conducting layer, in order to leave a portion of each of the first and second conducting layers exposed;
(e) locating a coated mesh material on top of the spacer layer and permanently securing it to the spacer layer;
(f) locating a second non-conducting substrate on top of the mesh material and permanently securing it in such a way as to leave an extended area of mesh exposed;
(g) applying a sample to the extended mesh area in order to fill or flood the device sensing area, by wetting of the mesh with sample; and
(h) quantifying the analyte in the sample by reaction with the reagent on the first conducting layer.
The electrode device allows the application of a small volume of sample (typically less than 2 xcexcL) to the mesh extension. This is followed by flooding of the device sensing area with sample, bringing it into intimate contact with the measuring electrodes. The cavity may be filled either by placing a drop of sample liquid on top of the exposed mesh at the edge of the cavity or by contacting the edge of the cavity with the sample.